Time-Temperature Superposition WLF-Equation

The amounts by which the curves where shifted are represented by 

Although the results in Fig. 1.32 where shifted to a reference temperature of 298 K (25°C), Williams, Landel and Ferry [14] chose Tref = 243K for 

Stress relaxation master curve. For the poly-a-methylstyrene stress relaxation data in Fig. 1.33 [8], create a master creep curve at Tg (204°C). Identify the glassy, rubbery, viscous and viscoelastic regions of the master curve. Identify each region with a spring-dashpot diagram. Develop a plot of the shift factor, log (ax) versus T, used to create your master curve log (ot) is the horizontal distance that the curve at temperature T was slid to coincide with the master curve. What is the relaxation time of the polymer at the glass transition temperature  

The master creep curve for the above data is generated by sliding the individual relaxation curves horizontally until they match with their neighbors, using a fixed scale for a hypothetical curve at 204°C. Since the curve does not exist for the desired temperature, we can interpolate between 208.6°C and 199.4°C. The resulting master curve is presented in Fig. 1.34. The amount each curve must be shifted from the master curve to its initial position is the shift factor, log (aT). The graph also shows the spring-dashpot models and the shift factor for a couple of temperatures. 

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